Spatiotemporal slip distribution associated with the 2012–2016 Tokai long-term slow slip event inverted from GNSS data

We used Global Navigation Satellite System (GNSS) time series data to estimate the spatiotemporal slip distribution for a long-term slow slip event (L-SSE) that occurred in the Tokai region, central Japan, from 2012 to 2016. Since all the used GNSS data were affected by the postseismic deformation associated with the 2011 Mw9.0 Tohoku-Oki earthquake, we removed such postseismic signal from the time series of three components at each of the stations. The minimal time window for an inversion analysis was set to 0.5 years (6 months), taking into account the signal-to-noise ratio of displacements for each time window. In the horizontal displacement fields, displacements were observed in the south‒southeast and southeast directions on the west and east sides of Lake Hamana, respectively, with temporal changes in their amounts and directions. In the vertical displacement fields, uplift was observed on the east side of Lake Hamana. From these data, we estimated the L-SSE initiated in approximately 2012.5 and ended by 2017.0, indicating the duration time is 4.5 years and the duration was much longer than that obtained in a previous study. Using these data, we performed the inversion analysis, in which three a priori information were assumed, i.e., the spatial distribution of slip is smooth, slip mainly occurs in the direction of plate convergence, and the temporal variation in the slip is smooth, to obtain the spatiotemporal slip distribution on a plate boundary with 3-D geometry. As a result, we identified that the L-SSE consisted of two subevents. The first subevent initiated on the southwest side of Lake Hamana and expanded during the period from 2013.0 to 2014.5. The maximum slip velocity during the period from 2012.5 to 2017.0 was estimated to be approximately 3.5 cm/year there for 2013.5–2014.0. The second subevent took place on the west side of Lake Hamana gradually from 2015.0 to 2015.5, continued, and expanded from 2015.5 to 2016.5. From the cumulative slip distribution, we found that its shape spread in the dip direction and obtained a maximum slip of approximately 10.6 cm, a moment release of 2.7 × 1019 Nm, and an equivalent moment magnitude of 6.9. Comparing our results with the L-SSE that occurred in the Tokai region between 2000 and 2005, we found that the slip initiation location was almost the same, but the subsequent slip location was more southerly for the 2012–2016 Tokai L-SSE. Additionally, the maximum slip velocity and moment magnitude were smaller for the 2012–2016 L-SSE.